The United States issued a firm condemnation of the shelling of a United Nations school in Gaza that killed at least 16 Palestinians on Wednesday, but also confirmed it restocked Israel’s dwindling supplies of ammunition.

The White House expressed concern that thousands of civilians who had sought protection from the UN were at risk after the shelling of the girls’ elementary school. Some 3,300 civilians were taking shelter there, after being told by Israel to leave their homes.

The UN agency for Palestinian refugees, UNRWA, which runs the school, said its initial assessment was that it has been struck by Israeli artillery.

“The United States condemns the shelling of a UNRWA school in Gaza, which reportedly killed and injured innocent Palestinians – including children – and UN humanitarian workers,” said Bernadette Meehan, a spokeswoman for the White House’s National Security Council.

“We are extremely concerned that thousands of internally displaced Palestinians who have been called on by the Israeli military to evacuate their homes are not safe in UN designated shelters in Gaza.”

Meehan and other US officials that condemned the attack did not specifically accuse Israel of responsibility for the shelling, saying there were conflicting reports about the circumstances of the incident that required further investigation. They did not specify the nature of those conflicting reports.